Selective reduction of NO2 with acetaldehyde (CH3CHO) was investigated over alumina-supported catalysts. Among the catalysts tested, Co/Al2O3 showed the highest activity with a maximum performance at 2wt.% Co loading. The comparison of the activity of Co/Al2O3 and Al2O3 for several unit reactions suggested that one of the roles of supported Co is to suppress the combustion of CH3CHO by O-2, resulting in an enhancement of the selective reaction of CH3CHO with NO2. The observation of the adsorbed species by in situ FT-IR spectroscopy showed that enolate species acts as the intermediate for NO2 reduction by CH3CHO and that another role of Co is to promote the production of the enolate species. (c) 2006 Elsevier B.V. All rights reserved.